Synergizing Fe2O3 nanoparticles on single atom Fe-N-C for nitrate reduction to ammonia at industrial current densities

Abstract

The electrochemical reduction of oxidized nitrogen species enables a pathway for the carbon neutral synthesis of ammonia (NH3). The most oxidized form of nitrogen, nitrate (NO3-) can be reduced to NH3 via the nitrate reduction reaction (NO3RR), which has been demonstrated at high selectivity. However, to make NH3 synthesis cost-competitive with current technologies, high NH3 partial current densities (jNH3) must be achieved to reduce the levelized cost of NH3. Here, we leverage the high NO3RR activity of Fe-based materials to synthesize a novel active particle-active support system with Fe2O3 nanoparticles supported on atomically dispersed Fe-N-C. By synergizing the activity of both nanoparticles and single atom sites, the optimized 3xFe2O3/Fe-N-C catalyst demonstrates an ultrahigh NO3RR activity, reaching a maximum jNH3 of 1.95 A cm-2 at a Faradaic efficiency (FE) for NH3 of 100% and an NH3 yield rate over 9 mmol hr-1 cm-2. Operando XANES and post-mortem XPS reveal the importance of a pre-reduction activation step, reducing the surface Fe2O3 (Fe3+) to highly active Fe0 sites, which are maintained during electrolysis, to realize the ultrahigh NO3RR activity. Durability studies demonstrate the robustness of both the Fe2O3 particles and Fe-Nx sites at highly cathodic potentials, maintaining a current of -1.3 A cm-2 over 24 hours, a near unity FENH3. This work exhibits an effective and durable active particle-active support system enhancing the performance of the NO3RR, enabling industrially relevant current densities and near 100% selectivity.